Electron discharge device



Feb. 8, 1938. c. s. BULL ET AL ELECTRON DISCHARGE DEVICE Original Filed Dec. 24, 1934 N m Mm 0 2 W as m m/ 0 1% A Q Patented Feb. 8, 1938 ELECTRON DISCHARGE DEVICE Cabot Seaton Bull, Hillingdon, and Sidney Rodin,

Enfleld, England, assignors to Electric and Mnsica-l Industries Limited, Middlese Ensland, a company of Great Britain Application December 24, 1934, Serial No. 759,008.

Renewed September 18 January 8, 1934 1937. .In Great Britain 16 Claims. (Cl. 25027.5)

The present invention relates'to electron discharge devices and more particularly to thermionic valves of the kind commonly known as tetrodes having an anode, a cathode and, between the anode and cathode, two grid electrodes.

It is known that valves of this kind tend to have their performance adversely affected by secondary electron emission from the anode to the screen grid electrode (that is the grid electrode nearer the anode) when, in operation, the anode potential falls below that of the screen id. In order to reduce or prevent such secondary emission it has been proposed to make use of an additional grid between the screen anode.

Another way in which secondary emission can be reduced or prevented is to cause a space charge to be. set up in the space between the anode and screen grid and it is an object of the present invention to provide a tetrode of this kind in which the anode may act as part of the envelope of the discharge device. Discharge tubes having an anode acting as part of the envelope and methods for making such tubes are for example described in United States patent specifications numbered 1,914,634, 1,942,824 and1,965,408 and in British patent specifications numbered 369,052, 370,174,

371,992 and 389,170, and many of the features and methods described in these specifications can with advantage be applied to valves according to the present invention.

According to the present invention, in an electron discharge device comprising, in the, order named, a cathode, acontrol grid, a screen grid" and an anode, there is provided in the space between the screen grid and anode one or more shielding members extending over a considerable portion of the anode and adapted to shield from the cathode a substantial fraction of the total surface of the anode which is directed towards the cathode. For example, when the anode surrounds the cathode, two shielding members may be provided and each may extend about one quarter of the way around the anode surface and together the members may shield from 'the cathode about one half of the total anode surface directed towards the cathode. The shielding members may be of insulating material and may be arranged in contact withthe anode. Alternatively the members may be spacedirom the anode and in that case they may be of conducting material. If the shielding members are of insulating'material they may be wholly or partly coated with conducting material, this conducting coating being in any case out of electrical contact grid and with the anode. If the shields are of insulating material, whether coated with conducting material or not, they become charged by the electrons and assume a potential approaching that of the cathode. I1 the shields are of conducting material or have a conductive coating they may be connected with a point of low potential, for example the cathode, or they may be left unconnected. In any case the shields assume a low potential relative to the anode and serve to direct the electron stream on to an unshielded part of the anode surface.

According to a further featureof the present invention, in a discharge device comprising a cathode surrounded, in the order named, by a control grid, a screening grid and an anode, in which the anode forms part of the envelope of the device, one or more shielding members are disposed between the anode and the screening grid, these shielding members extending over a substantial fraction of the anode surface and serving substantially to prevent electrons from reaching the anode over this part of its surface.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawing, wherein the Figures 1, 2, and 3 represent sections of electron discharge devices according to this invention.

Like parts in the various figures are indicated by like references.

In all the figures of the drawing a metal cylinder l is arranged to form part oi the envelope of a discharge device and to act as anode. Within, the anode I and coaxial therewith is arranged a rod shaped cathode 2. The cathode is surrounded by a control grid 3 which is supported by two rods 4, 5 lying in a diametralplane. The grid 3 may be formed of ,wire wound helically upon the rods 4, 5 and is diamond shaped, the four corners being rounded and the longer diagonal lying in the plane of the supporting rods. Around the control grid 3 is a screening grid 6 which may of the anode.

In Fig. 2 the shielding members take the form of pieces of glasstube ll, I2 01: diameter ap- 2. An electron discharge device having an proaching the distance between the rods I, 8 and In addition to the'shielding members shown in the anode I. The tubes Ii, I2 are mounted with evacuated envelope and arranged within said en-" velope, in the order named, a cathode capable of electron emission, a control grid, a screen grid,

an anode, and shielding means impermeable to .electrons'and of insulating material and disposed Fig. 1, insulating shielding members l3, M in contact with the anode may be inserted as shown in Fig. 3. I

The shielding members are of such size as to shield a considerable fraction, preferably about one-half, of the anode surface from the cathode emission.

If the spacing of the electrodes is suitably chosen for the conditions under which the discharge tube is to'work, it will befound that a space charge is setup in that part of the space between the cathode and the anode which is not shielded by the glass tubes or rods, the tubes or rods becoming charged by impinging electrons and serving to deflect electrons intothis part of the space. The space charge serves to prevent secondary'electron emission from the anode to the screening grid or at least to reduce such secondary emission.

The shielding membersmay'be coated with' conducting material which may be connected to a point of relatively low potential, for example to the cathode. Preferably, however, if the conductive coating is applied, it is not connected electrically but is allowed to assume a'working' potential by electron bombardment. Where two shielding members are provided on each side, as shown in Fig. 3, conductive coatings may be ap ,plied to the outersurfaces of the inner shields 9, l0 and the inner surfaces of the outer shields l3, IA. The coating may be of silver and may be applied by providing a number of silver pellets within the tube, for example upon the screen grid. When the valve is being gettered the silver melts and condenses upon the shields or at least upon parts thereof. When a shield in contact with the anode is to be coated in this way,it is preferably undercut at its edges as shown at IS in Fig. 3 in such a way as to make the surface in contact with the anode of smaller area than the opposite surface. In this way it is ensured that the coated surface remains insulate-d from the anode. h

In some cases shields of --.conducting material, arranged out of contact with the anode, may be used.

r The above description is given by way of example only and many modifications of the in- 5 vention will be apparent to those versed in the 60 for example of glass.

art.

For example, the'anode need not always form to shield from said cathode substantially onehalf of the total surface of'sa'id anode which is directed towards said cathode.

3. An electron discharge device having an evacuated envelope and arranged within said envelope, in the order named, a cathode capable of electron emission, a control grid, a, screen grid, shielding means and an anode, said anode having a surface directed towards said cathode and said shielding means comprising at least one member of insulating material coated with conducting material on the surface thereof directed toward said cathode and extending over a substantial fraction of said anode surface.

4. An electron discharge device having an evacuated envelope and arranged within said envelope, in the order named, a cathode capable of electron emission, a control grid, 2. screen grid, shielding means and an anode, said shielding means being in the form of a coating of insulating material upon the surface of said anode directed towards said cathode and extending over a substantial fraction of said surface.

5. An electron discharge device having an evacuated envelopeand arranged within said envelope, in the order named, a cathode capable of electron emission, a control grid, a screen grid, shielding means and an anode, said shielding cathode coated with conducting material and an opposite surface in contact with a substantial fraction of the. surface of said anode which is directed towards said cathode, the edge surfaces of said insulating material being undercut to make the area of the second named surface "smaller than that of the first named surface.

'7. An electron discharge device having an evacuated envelope and arranged within said envelope, in the order named, a cathode capable of electron emission, a control grid, 2. screen grid, a

a part of the envelope but may be a suitablev shaped electrode arranged within an envelope Further, cathodes of other than rod shape may be employed and the grids may be of circular, oval or other suitable shape in plan view. 0ther suitable methods of supporting the grids may be employed.

We claim:

1. An electron discharge device having an evacuated envelope and arranged within said envelope, in the order named, a cathode capable of electron emission, a control grid, a screen grid,

70 an anode forming part of said envelope and having a surface directed towards said cathode and shielding means having a closed surface of substantial area impermeable to electrons and being disposed to shield from said cathode substantially 7'5 one-half of said anode surface.

first shield member, a second shield member and an anode, said shield-members extending over a substantial fraction of the surface of said anode directed towards said cathode, and said shield members having surfaces facing one another, these surfaces being coated with conducting material.

8. An electron discharge device having an evacuated envelope and arranged within said envelope, in the order named, a cathode capable of electron emission, a control grid, a screen grid, shielding, means and an anode, said shielding means being substantially impermeable by electrons and extending over substantially one half of the surface of said anode directed towards said cathode. V

9. An electron discharge device -comprising a and said anode, said shielding means having a closed. surface of substantially the same length as the axial length of the active surface of said anode and of a width to shield substantially one-half of said active surface.

10. An electron discharge device comprising a cathode, a control grid, and an anode surrounding and coaxial with said cathode. a screen grid coaxial with and interposed between said control grid and said anode and having a supporting rod, and a shield of insulation and impermeable to electrons disposed between said supporting rod and said anode.

11. An electron discharge device according to claim 10 wherein the surface of said shield facing said cathode is coated with conducting material.

12. An electron discharge device according to claim 10 wherein said shield is in the form of a coating of insulation on the surface of said anode facing said cathode.

13. An electron discharge device according to claim 10 wherein said shield of insulation has an inner surface facing said cathode and an outer surface in contact with said anode, the edges of said insulation being undercut to make the area of its outer surface less than that of its inner surface. y

14. An electron discharge device according to claim 10 wherein said shield of insulation is cylindrical in form.

15. An electron discharge device comprising a cathode, an anode surrounding said cathode, a control grid and a screen grid coaxial with and interposed between said cathode and said anode, and a pair of shields mounted between said screen grid and said anode on opposite sides of said screen grid and extending over the axial length of the active surface of said anode and each of a width toshield substantially one-fourth of the ac- 16. An electron discharge device comprising a cathode, a grid surrounding said cathode. a second grid surrounding and coaxial with said cathode and having a pair of diametrically opposite grid support rods, an anode surrounding said second grid, and-apair of arcuate shield strips 

